Deposits, said plainly
Serious agencies take deposits, and this desk does too, because a deposit protects both sides of the door. You know the hour is held for you. She knows the booking is real before she crosses London for it. For a standard hotel or residence visit that means 25 percent by bank transfer, sent after the desk has checked three things on WhatsApp: your full name, the hotel, the room.
Overnights, events and travel bookings are planned further ahead and keep her diary clear for longer, so they are secured with 50 percent upfront. The travel component of that payment is non-refundable. The rest follows in cash on the night, or by transfer ahead if that suits you better.